1. Field of the Invention
The present invention relates generally to printers and, more particularly, to an image forming method and apparatus having a photoreceptor web which can be directly heated.
2. Description of the Related Art
In general, an image, forming apparatus such as a liquid electro-photographic printer, as shown in FIG. 1, includes a photoreceptor web 10 that circulates and is supported by a plurality of support rollers 11, and a plurality of laser scanning units 12, a plurality of development units 13, a drying unit 17 and a transfer unit 19, which are sequentially arranged in a direction that the photoreceptor web 10 proceeds. The laser scanning units 12 and the development units 13 are alternately installed. The laser scanning units 12 scan light onto the photoreceptor web 10 to form an electrostatic latent image. Each of the development units 13 develops the electrostatic latent image with a predetermined developer to form a developed image, and includes a developing roller 14 for developing the electrostatic latent image, and a squeegee roller 15 for squeegeeing the developed developer so that the liquid carrier is removed from the developer. The drying unit 17 includes a drying roller 16 for drying the carrier remaining on the photoreceptor web 10. The transfer unit 19 includes a transfer roller 18 for transferring the image passing through the transfer unit 19 to a sheet of print paper P.
In the above structure, the developer that is developed by the development unit 13 comprises ink which is a mixture of a powdered toner having a predetermined color and a liquid carrier that works as a solvent. The developer is supplied to a development gap G between the photoreceptor web 10 and the developing roller 14. The developer is moved to the electrostatic latent image due to a difference between the level of electric potential applied to the developing roller 14 and that of the electrostatic latent image. Toner of the ink used for the development is made filmy by the squeegee roller 15 and becomes a toner image while most of the carrier is squeegeed and removed. The small amount of carrier remaining on the photoreceptor web 10 is dried by the drying unit 17.
In the above image forming process, the efficiency of squeegeeing the carrier with the squeegee roller 15 greatly affects not only the quality of the toner image which is formed to be filmy, but also the performance of the drying roller 16 and the transfer roller 18. For example, since the amount of carrier to be dried by the drying unit 17 decreases as the efficiency of squeegeeing by the squeegeeing roller 15 is improved, the time for drying carrier can be reduced. When the carrier drying time decreases, the number of consecutively printed papers increases. Also, the life span of the drying roller 16 and the transfer roller 18 extends.
Studies to improve the efficiency of squeegeeing have been widely made. There has been a recent report that the efficiency of squeegeeing is closely related to the glass transfer temperature Tg of ink or temperature of the photoreceptor web 10. It is widely known that a glass transfer temperature value of xe2x88x921 Tg is an appropriate value. The efficiency of squeegeeing increases as the value is lower than xe2x88x921 Tg to a degree. However, according to the results of various experiments, when the glass transfer temperature is lowered under xe2x88x9210 Tg, many problems are generated. For example, there is wash-off in which the filmy toner image is developed by another developing unit for another color and partially lost; dry picking in which the toner image is tore off by the drying roller 16; and transfer error in which the toner image is not completely transferred to the print paper and remains on the transfer roller 18 as a latent image.
A method to improve the squeegeeing efficiency is to constantly maintain the temperature of the photoreceptor web 10. For this purpose, a conventional heating apparatus is used which includes an additional heating means (not shown) for indirectly heating the photoreceptor web 10, a temperature detecting sensor (not shown) for measuring the temperature of the photoreceptor web 10, and a means for preventing overheating. However, the apparatus has drawbacks because the number of parts is increased which complicates the structure and raises the cost. Also, since the heat loss is continuously generated from the photoreceptor web 10 due to the ink, the temperature of the photoreceptor web 10 cannot be constantly maintained.
To solve the above problems, it is an objective of the present invention to provide a photoreceptor web and an image forming apparatus and method having an improved structure in which the temperature of the photoreceptor web can be constantly maintained by directly heating the photoreceptor web so that the efficiency of squeegeeing and drying is improved.
Accordingly, to achieve the above objective, there is provided a photoreceptor web for an image forming apparatus which comprises a base layer having a front and rear surface, a conductive ground layer formed on the front surface of the base layer, and a photosensitive layer formed on the ground layer, where an electrostatic latent image is formed by scanned light. The invention further includes a first protective layer formed on the photosensitive layer, which protects the photosensitive layer, and a heating layer formed on the rear surface of the base layer in a predetermined pattern, which receives electric power and generates resistance heat, wherein the photoreceptor web is maintained at a constant temperature by the resistance heat generated by the heating layer.
It is preferred in the present invention that the first protective layer comprises a barrier layer formed on the photosensitive layer, which prevents intrusion of foreign materials into the photosensitive layer, and a release layer formed on the barrier layer, where an image is formed.
It is also preferred in the present invention that a pair of power terminals, which receive the electric power, are exposed at each of both side portions of the heating layer.
It is also preferred in the present invention that the photoreceptor web further comprises a second protective layer coated or positioned on a surface of the heating layer to prevent damage to the heating layer.
Further, it is preferred in the present invention that the second protective layer is formed of a material selected from the group consisting of silicon, polyamide and polyethylene.
To achieve another aspect of the present invention, there is provided an image forming apparatus which comprises a photoreceptor web supported by a plurality of support rollers, which is operative to circulate along a predetermined path, a laser scanning unit, positioned proximate to the photoreceptor web, which scans light onto the photoreceptor web to form an electrostatic latent image, a developing unit, which develops the electrostatic latent image using a developer to form an image, a drying unit, which dries the photoreceptor web, a transferring unit, which transfers the image formed on the photoreceptor web to a printable medium, such as a sheet of print paper, and a power supply unit, which supplies power, which heats the photoreceptor web. Here, the photoreceptor web comprises a base layer, a conductive ground layer formed on the base layer and a photosensitive layer formed on the ground layer, where an electrostatic latent image is formed by the light scanned by the laser scanning unit. A first protective layer is formed on the photosensitive layer, which protects the photosensitive layer, and a heating layer is formed on a rear surface of the base layer in a predetermined pattern, which receives the power from the power supply unit and generates resistance heat, wherein the photoreceptor web is maintained at a constant temperature by the resistance heat generated by the heating layer.
In a preferred embodiment according to the present invention, the power supply unit comprises a support bracket installed adjacent to the photoreceptor web, a conductive roller rotatably installed at the support bracket to be passively rotated in contact with the side portion of the heating layer, a conductive elastic bar supported at the support bracket that elastically presses the conductive roller toward the heating layer, and a power source electrically connected to the elastic bar.
It is preferred in the present invention that the power supply unit comprises a support bracket installed adjacent to the photoreceptor web, a brush member supported at the support bracket and having a conductive brush which elastically contacts an exposed side portion of the heating layer, and a power source electrically connected to the conductive brush for supplying an electrical signal.
It is also preferred in the present invention that the photoreceptor web further comprises a power terminal which is exposed from both side portions of the heating layer and is electrically connected to the power supply unit.
It is also preferred in the present invention that the photoreceptor web further comprises a second protective layer coated or positioned on a surface of the heating layer to prevent damage to the heating layer.
Also, it is preferred in the present invention that the second protective layer is formed of a material selected from the group consisting of silicon, polyamide and polyethylene.
A method of forming an image on a heat-maintained photoreceptor web is also contemplated and comprises rotating the photoreceptor web by a plurality of support rollers, supplying power to a heating layer of the photoreceptor web by continuously moving the photoreceptor web in contact with a power source and generating resistance in the heating layer using the supplied power so that heat is produced in the heating layer. The method also includes maintaining the heat in the heating layer wherein the photoreceptor web is maintained at a constant temperature, scanning light onto the photoreceptor web to form an electrostatic image, and developing the electrostatic image using a developer to form an image. The photoreceptor web is then dried and the image formed on the photoreceptor web is transferred to a printable medium.